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March 26, 2012, 06:24 
manual integration of pressure and wall shear on rotating wall

#1 
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Max
Join Date: May 2011
Location: old europe
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Hi,
i am studying the flow around a rotating cylinder. I'm interested in lift/drag forces and the torque that act on the cylinder. I used the functions force_x, force_y and torque_z. My results showed that the rotational velocity of the cylinder has almost no influence on the torque applied to the cylinder by the fluid. To me that seems unlikely in a very viscous flow with no slip boundary conditions  The cylinder Reynolds number is in the magnitude of 1. Though the documentation states that the force and torque functions sum up the visoucs and pressure forces, I am not sure if they really do what I want, particularly in this special case with rotating boundaries. That's why I tried to manually integrate the viscous and pressure forces around the cylinder to be able to seperately look at each one of them. My manually calculated values differ significantly from the values obtained by the force and torque commands. drag force: force_x()@CYLINDER = 0.7 [N] areaInt_x(Pressure)@CYLINDER + areaInt(Wall Shear X)@CYLINDER = 2.1 [N] torque: torque_z()@CYLINDER = 2.3 [J] areaInt(Wall Shear Y*x)@CYLINDER + areaInt(Wall Shear X*(y+4.32 [m] ))@CYLINDER + areaInt_y(Pressure*x)@CYLINDER + areaInt_x(Pressure*(y+4.32 [m] ))@CYLINDER = 0.63 [J] or with a local coordinate frame (CYL): areaInt_x_CYL(Wall Shear X*(y))@CYLINDER + areaInt_y_CYL(Wall Shear Y*x)@CYLINDER + areaInt_x_CYL(Pressure*(y))@CYLINDER + areaInt_y_CYL(Pressure*x)@CYLINDER = 8.9 [J] Here are my questions:  Why do the values differ?  The manual torque calculation using a local coordinate frame requires a axis specification. So I used areaInt_y_CYL(Wall Shear Y) instead of areaInt(Wall Shear Y). I guess the integration without axis definition is physically more correct...is there a way to use the areaInt function on a local coordinate frame without specifying an axis? Any help will be appreciated! 

March 26, 2012, 08:26 

#2 
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Raja_Bhai
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What value do you get for areaInt_x(Pressure)@CYLINDER?


March 26, 2012, 09:33 

#3 
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Max
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areaInt_x(Pressure)@CYLINDER = 1.97 [N]


April 3, 2012, 08:01 

#4 
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Max
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Anyone got an idea?


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